Mandatory Weeping Tile Disconnection to Reduce the Impact of Basement Flooding

Transcription

1 Mandatory Weeping Tile Disconnection to Reduce the Impact of Basement Flooding May 26, 2011 Kyle Chambers, P.Eng. Environmental and Engineering Services Department City of London x5854

2 Outline Terminology Background: Inflow/Infiltration; Basement flooding Case Study: Sherwood Forest Subdivision Causes of Basement Flooding Public and Private Side Solutions Next Steps

3 Terminology

4 What is I/I? Inflow and Infiltration (I/I) Storm water and/or ground water entering sanitary sewers Why is it a problem? Higher conveyance costs at pumping stations Higher treatment costs at pollution control plants Combined sewer overflows, bypasses at plants Basement flooding

5 Where does I/I come from? Cracks in pipe Combined sewers (catchbasins, cross connections) Downspout connections to sanitary Weeping tile connections to sanitary

6 London s Weeping Tile History Prior to 1985: weeping tile connected to sanitary sewer : weeping tile connected to sump pit; sump pump discharge to surface 1995-present: weeping tile connected to sump pit; sump pump discharge to storm sewer PDC

7 Sump Pump Diagram Courtesy of ICLR

8 Basement Flooding Problem within the City of London for many years Can be caused by: 1) Isolated issues leading to localized basement flooding 2) Systemic issues leading to regional basement flooding

9 Basement Flooding Isolated issues can include: Private Drain Connection (PDC) collapse/blockage Weeping tile blockage Poor / altered lot grading (sloping towards house) Basement wall deterioration (cracks, etc) Downspout discharge location (adjacent to building)

10 Basement Flooding Regional issues can include: Blocked Sanitary Main Surcharged/Overloaded Sanitary Main Major source of Surcharged Sanitary Main: Weeping tile connected to the sanitary sewer, plus: Lot grading sloped towards house; and/or, Downspout discharge location (at or near foundation wall). Leading to: sanitary surcharging from excess stormwater (I/I) Resulting in: Basement Flooding

11 Case Study: Sherwood Forest Subdivision developed late 1970s to early 1980s Historical basement flooding prone area Hit particularly hard May 28, 2009: 83mm of rain in 5 hours

12 Flooding Areas May 28/09

13 Flow Monitors Rainfall

14 Flow Monitors

15 What s Happening? Clay soil does not absorb water Lot grading has settled over the years Zero lot line homes Closely spaced together = high % of roof and hard surface Poorly placed downspout discharge location

16 Downspouts

17 Aerial Photos

18 Area Study City engaged consultant to: Analyze flow monitoring data Build and calibrate computer model Prepare basement flooding mitigation solutions: o Public alternative (City infrastructure) o Private alternative (Homeowner property) Two surveys administered to all 2,000 residences one survey conducted by City of London one survey conducted by ICLR

19 Public Side Alternative City Infrastructure Improvement: Pipe upsizing and storage Protects basements up to the 25-year design storm event Estimated cost: $10 million (100% funded by City) Risk of larger storm, or back to back storms: overwhelming storage and conveyance more basement flooding This option only accommodates the I/I

20 Pipe Upsizing and Storage 2,300m of Pipe Upsizing 1400m 3 of Inline Storage

21 Private Side Alternative Private Homeowner Improvements: Disconnect weeping tile from private sanitary lateral to remove I/I Modeling analysis indicates 160 homes required to disconnect Protects basements beyond 25-year design storm event Estimated cost: $2 million (100% funded by City) Includes: weeping tile disconnection, sump pump, backwater valve, dedicated storm PDC for sump pump discharge, and road resurfacing This option removes the I/I at the source

22 Private Side Alternative Current State (25-year Storm): 70% Home Disconnection (25-year Storm):

23 Private Side Alternative: Advantages Disconnecting the weeping tile from sanitary: Removes I/I at the source Saves treatment costs Reduces risk of overwhelming system with next big storm Significantly more cost effective: only 20% of public side alternative

24 How? Voluntary (Current Practice): 75% Grant Program for Basement Flooding Protection (includes sump pump, backwater valve, weeping tile disconnection, storm PDC) Very low uptake; dry basement homeowners have no interest, even though they are contributing. Mandatory (Proposed): 100% funded by City Focus on problem areas only

25 Obstacles Public Perception: City approved original subdivision therefore City responsible to fix City sewer pipes are too small Nearby development is causing our flooding Homeowner reluctance to retrofit sump pump: Noise, nuisance Electricity and maintenance costs

26 Next Steps Currently preparing Council report which recommends: Focused, mandatory weeping tile disconnections All sump pumps to discharge directly to storm PDCs to prevent icing problems experienced in other areas Council acceptance will be key factor

27 Thank You